I'm in pretty complete agreement with the previous posters in this thread (partic. A-man, Straightline, and DonnyB.), but wanted to add one thought that's pretty obvious, but lots of people seem to forget it:
Any half-way decent skier can ski most snow conditions on any pair of boards. You can ski deep powder or crud on 207 Renntigers, 188 p40's or 163 shorty slaloms. It may not feel as good as if you were on an optimal set of boards, but it can be done, and its not even all that hard. As you get closer to the optimal type of skis, it feels better and it gets easier, but you don't have to agonize over selecting THE optimal pair for yourself.
That said, as applied to this thread, an 83 mm Rex will feel quite a bit better in deep snow than Seth's SX11, especially if you are on the lighter side. A 90+ ski will feel still better yet, and will allow him to do go both faster (with stability) and slower (without sinking in and bogging down), especially if you are heavier than average.
IMHO, Seth's real decision comes down to whether he wants to spend the more frequently occurring 4 - 6 inch days on skis (90+ models) that might be a bit overkill for the situation but will still work fine, or on mid-80mm skis that are "perfect" for this situation, but which will leave a performance gap on the really deep days that occur less frequently (ie, Descender's spot-on comment).
I didn't notice any mention of Seth's weight, but my recommendation to him (or anyone else) would be that if he is over 185-190 lbs, go with 90+ mm skis as his second pair. Much under 160, go with the Rex's, and in the middle, make a decision based on the pros and cons other posters have given.
My personal fat ski situation is this - For the past 4 or 5 years I've been on a 190 Explosiv (95mm), and a 184 10ex (83mm) for the past 2 seasons. If I only feel like taking two pairs with me, and deep snow is a serious possibility, I take a pr of groomer skis and the Explosivs. If I know its going to be a bit soft but not deep, the Explosivs stay home and I ski the 10ex's. If I have no idea what the snow is going to be like, I prefer to err on the side of FATness.
My opinion is that getting caught having to ski 24 inches of cut-up, set-up crud on 80 mm skis is worse than "getting caught" skiing 4" of new on 95 mm powder boards. The latter make the 4" feel like velvet. YMMV.
Tom / PM